1. Field of the Invention
Aqueous starch dispersions have a number of practical applications as thickening agents, particularly in foods. Altering the rheological properties of starch dispersions would enhance the usefulness of starch in these areas. Also, the formation of rigid gels from cooked starch pastes is a major problem that limits the use of native unmodified starch in certain applications requiring storage after cooking. An effective method for reducing or eliminating this undesirable gelling, or changing the properties of these gels to make them more suitable for food applications, would greatly broaden the range of starch applications.
This invention relates to compositions of starch that have unique rheological and gelling properties. These properties are the unexpected result of: (1) the addition of minor amounts of natural gum to the system and (2) the cooking procedure, which renders both polysaccharides totally soluble, reduces polysaccharide molecular weight, and yields the most intimate mixture possible between starch and the natural gum additive.
2. Description of the Prior Art
Starch is a high molecular weight polymer composed of repeating 1,4-.alpha.-glucopyranosyl units (anhydroglucose units, or AGU) and is typically a mixture of linear and branched components. The linear component, amylose, has a molecular weight of several hundred thousand; while the molecular weight of the branched amylopectin is on the order of several million. Although normal cornstarch contains about 20-25% amylose, cornstarch varieties are available commercially that range in amylose content from 0% (waxy cornstarch) to about 70% (high-amylose cornstarch).
Starch occurs in living plants in the form of discrete granules ranging from about 5 to 40 microns in diameter, depending on the plant source. It is well known that starch, as isolated from the plant in its native state, is insoluble in water at room temperature because of strong hydrogen bonding between polysaccharide macromolecules. Areas of crystallinity within starch granules also inhibit water solubility. When a water suspension of granular starch is heated, granules at first slowly and reversibly take up water with limited swelling. Then, at a definite temperature, which is typically about 70.degree. C., granules swell rapidly and irreversibly; and areas of crystallinity within the granule are lost. The temperature at which this occurs is commonly referred to as the gelatinization temperature.
Near the gelatinization temperature, a measurable percentage of the starch, in particular the amylose component, becomes soluble and diffuses out of the granule matrix and into the surrounding water. As the temperature is increased beyond about 70.degree. C., a greater percentage of the starch becomes soluble, and the granules become highly swollen and partially disrupted, until, at a temperature of about 90.degree.-100.degree. C., a viscous dispersion of starch in water is obtained. However, despite this outward appearance of solubility, starch is only partially water soluble and exists largely as highly swollen granules and granule fragments that are easily separable from starch solution, for example, by centrifugation. In fact, when cornstarch is heated in water at 95.degree. C., only about 25% of the starch actually dissolves, the remainder being present as swollen granules and granule fragments.
True solutions of starch in water, with no remaining granules and granule fragments are difficult to prepare using conventional cooking techniques, but can be readily prepared by passing starch-water slurries through a continuous steam jet cooker. Jet cooking has been used commercially for decades to prepare starch solutions for nonfood applications, for example, in the paper industry. The method involves pumping an aqueous starch slurry through an orifice where it contacts a jet of high-pressure steam. Unlike conventional cooking, which tends to preferentially solubilize the amylose component, steam jet cooking dissolves amylopectin as well as amylose. Somewhat higher starch concentrations than those desired in the final dispersion are used to allow for dilution of the cooked dispersions with condensed steam.
There are basically two types of starch jet cookers used commercially, and these are discussed in an article by R. E. Klem and D. A. Brogly, Pulp & Paper, vol. 55, page 98-103 (May 1981). The first of these provides a process that is referred to as thermal jet cooking. In this process, the amount of steam added to the aqueous starch slurry is carefully controlled to achieve complete steam condensation during the cooking process. No excess steam is used. The second of these provides a process that is referred to as excess-steam jet cooking. In excess steam jet cooking, the steam entering the heating zone of the cooker exceeds the amount required to reach the desired cooking temperature. This excess steam in the heating zone thus acts to produce mechanical shearing of the starch and rupture of polysaccharide molecules, especially those having the highest molecular weight. This not only leads to total and complete polysaccharide solubility but also to a lower apparent viscosity of starch, as compared with starch processed by either thermal jet cooking or conventional batch cooking.
An inherent property of starch pastes obtained by standard cooking proedures is their tendency to form firm, rigid gels on prolonged standing. The tendency of starch pastes to gel increases with the amylose:amylopectin ratio in the granule. It is generally accepted that gel formation, i.e., retrogradation, is caused by aggregation of starch molecules through hydrogen bonding. Retrogradation and aggregation of the amylose chains occurs more readily than with amylopectin, because amylose is a straight chain polymer with little or no branching. However, under refrigeration, amylopectin will also aggregate over time and will contribute to the gel-forming property of starch.
Although the formation of firm, rigid gels from cooked starch dispersions may be desirable for some end-use applications, the formation of gels of this type is often undesirable in foods because of the deterioration of gel structure during storage. Steam jet cooking is thus not generally used in the food industry, since a totally soluble starch is obtained that can subsequently retrograde to form a gel. Moreover, jet cooked starch solutions that have been dried are often difficult to redisperse in water and generally do not yield lump-free pastes having the smooth consistency required for food applications.
Natural gums are plant are microbial polysaccharides or their derivatives that can be readily dispersed in either cold or hot water to produce viscous solutions [R. L. Whistler, in Encyclopedia of Polymer Science and Technology, vol. 11, page 403 (1969)]. Examples of natural gums that are widely used industrially are the seaweed polysaccharides such as carrageenan, the exudate gums such as gum arabic and tragacanth, the seed gums such as guar and locust bean, the microbial polysaccharides such as xanthan, and certain polysaccharide derivatives such as carboxymethyl cellulose (CMC). Although natural gums are widely used in the food industry as thickeners and as suspending agents, gums are expensive relative to starch; and they often must be imported from other countries, thus creating an uncertain source of supply.
References to starch-natural gum mixtures exist in the prior art, for example, U.S. Pat. Nos. 3,554,764; 4,192,900; 4,219,582; and 4,623,552. These mixtures have all been prepared by conventional cooking procedures, which do not produce a totally water-soluble starch. The usefulness of these prior art compositions as food additives is thus limited to freshly cooked preparations; since drying these compositions typically yields products that not only are difficult to redisperse in water but also produce lumpy rather than smooth dispersions.
U.S. Pat. No. 4,859,484 describes a 50:50 mixture of starch and a hydrocolloid gum such as guar. These compositions are prepared by: (1) hydrating each polysaccharide component separately with 0.7-2.0 parts of water for the starch and 0.2-2.0 parts of water for the hydrocolloid gum, (2) initimately mixing the two polysaccharides, and (3) passing the mixture through an extruder at elevated temperatures. Processing was carried out at the lowest possible level of added water and under conditions that cause minimum shear-induced and temperature-induced degradation of the final product. Starch was thus not totally solubilized in the process. Also, final products contain equal amounts of starch and hydrocolloid gum. There is no data in this patent to suggest and dramatic effect of a hydrocolloid gum additive, particularly guar, on the properties of starch when the polysaccharide of the system is totally solubilized and partially degraded.
Extrusion cooking of polysaccharides at high solids contents, as described in U.S. Pat. No. 4,859,484, provides a polysaccharide melt. References to the formation of starch melts are common in the published literature [see for example, Colonna et al., Journal of Cereal Science vol. 1, pages 115-125 (1983); and Stepto & Tomka, Chimia, vol. 41, pages 76-80 (1987)]. Evidence for melt formation in U.S. Pat. No. 4,859,484 is the observation that the semisolid, molten extrudate expands as it exits the extruder die and then rapidly hardens to form a rigid solid as the melt cools. Solidification of the melt on cooling is sufficiently rapid to allow the extrudate to be cut into small sections for further drying.
Starch melts having high water contents such as those formed in U.S. Pat. No. 4,859,484 shows glass transition temperatures (T.sub.g) that are not far above room temperature. Since annealing and reassociation of starch macromolecules occurs at temperatures above T.sub.g, the solubilities of extruded starch products tend to decrease on standing. An explanation of the effect of T.sub.g on starch properties is given in a chapter by Colonna et al. in "Extrusion Cooking," C. Mercier, P. Linko, and J. M. Harper, eds., American Association of Cereal Chemists, Page 262 (1989). In contrast, dried products prepared by the present invention are characterized by a low moisture content (about 5% or below) and a T.sub.g considerably above room temperature. As a result of the high T.sub.g the annealing of starch to form insoluble products is less likely to occur.
We are also aware of European Patent Application EP 366,898 (May 9, 1990) in which aqueous slurries of high amylose starch were jet cooked, apparently via the thermal jet cooking process, and solutions were then spray dried without allowing the steam pressure in the cooking system to be released. Unlike our process, starches were essentially undegraded, and they dispersed readily in water, yielding firm gels on standing. Although mixtures of starch and polygalactomannan gum were also processed in this manner, properties of these compositions were not dispersed. Also, there was no suggestion of the unique properties that can be configured upon starch by hydrocolloid gum additives, particularly guar, when excess-steam jet cooking is used and when solutions are drum dried at atmospheric pressure.